Electrical heating device



P 1968 SHIGERU HAYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE Filed Oct. 20, 1965 4 Sheets-Sheet 1 FlG.l

APPLIED VOUAGE CURRENT FLOW 3mm INVENTORS I SHIGERU HAYAKAWA TAKASHI l6U CHI YUKI O KASAHA RA WWWVM P 1968 SHIGERU HAYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE 4 Sheets-Sheet 2 Filed Oct. 20, 1965 FIG.3

304m .hzmmmzu I? T EMPERATURE 3mm INVENTORS SHIGERU HAYAKAWA TAKAS'IIYUKIO IGUCHI KASAHMA A I M M P 1968 SHIGERU HAVYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE Filed Oct. 20, 1965 4 SheetsSheet 5 F l G. 5

3mm INVENTQRS SH] GERU HAYAKNIA TAKASHI l6 UCHI YUKIO KASAHARA UnitedStates Patent Oifice 3,400,252 Patented Sept. 3, 1968 3,400,252ELECTRICAL HEATING DEVICE Sliigeru Hayakawa, Hirakata-shi, Osaka-fu,Takashi Iguchi, Kyoto-shi, Kyoto-fu, and Yukio Kasahara, Kadoma-shi,Osaka-fa, Japan, assignors t Matsushita Electric Industrial (30., Ltd.,Osaka, Japan Filed Oct. 20, 1965, Ser. No. 498,548 1 Claim. (Cl.219-504) ABSTRACT OF THE DISCLOSURE A temperature responsive resistancedevice which is responsive to change the flow of current therethrough atthe ends of the range of temperatures, so that it can be used as a selfcontrolled heating device, as part of a temperature control system, oras a temperaturecurrent switch. The device has a thermistor which has apositive temperature coeflicient of electrical resistance connected inseries of an external impedance, the series connected elements beingadapted to be coupled to a source of current and having a combinedtemperatureresistance characteristic which causes the currenttherethrough to drop abruptly when the temperature of the thermistorrises above the upper limit of the temperature range and to increaseabruptly when the temperature of the thermistor drops below the lowerlimit of the temperature range.

This invention relates to electric heating devices and the like, such asa Warming device, and particularly to an improved self-controllabledevice including a heating element which is temperature sensitive.

Prior art heating elements by themselves are difficult to control as tothe ambient temperature thereof and it is necessary to have additionalequipment for such control, such as a switch having a contact. However,such a contact, which turns electric current on or off depending uponthe ambient temperature, is apt to become oxidized, contaminated andwelded together over a long period of operating time and finally ceasesworking.

Recently, a thermistor with a positive temperature ooeflicient ofelectrical resistance has been discovered. This thermistor is a ceramicbody consisting essentially of barium titanate. A heating element madefrom this ceramic has a characteristic such that an electric currentflowing through the element decreases with increasing ambienttemperature and vice versa. Therefore, said heating element isresponsive to overheating. However, theam bient temperature controlledby the heating device varies with the heat capacity of the object ormaterial to be heated when the heating device comprises only heatingelements of the aforesaid ceramics. Furthermore, the range ofcontrollable temperature also varies with the heat capacity of theheated object or material.

The object of the present invention is to provide a heating device whicheliminates these drawbacks and which controls the ambient temperature ofthe object or material being heated within a given range of appropriatetemperatures while using no additional switch.

It is another object of the present invention to provide a temperatureresponsive control system which no longer requires an additional circuitopening and closing device.

It is a further object of the present invention to provide a noveltemperature current switch responsive to a temperature change to atemperature outside a range of temperatures.

For a better understanding of our invention reference may be had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of the temperature sensitive control systemof the present invention;

FIG. 2 is a graph showing the characteristic current vs. voltage curvesof the heating element operating at several ambient temperatures and thevoltage-current load line of the external impedance consisting ofreactance and resistance;

FIG. 3 is a graph showing variation in the current with the ambienttemperature for the heating element;

FIG. 4 is a circuit diagram of a temperature responsive control systemincorporating the temperature sensitive control of FIG. 1.

FIG. 5 is a cross sectional view of a milk warming device incorporatingthe temperature sensitive control system of the present invention;

FIG. 6 is a graph showing the temperature dependence of the electricalresistance of the thermistor; and

FIG. 7 is a graph showing the relation of current and temperature forthe device of FIG. 5.

Referring to FIG. 1, terminals adapted to be coupled to an alternatingcurrent source are connected to a heating element 1, which is athermistor with a positive temperature coefiicient of electricalresistance, and to an external impedance such as a reactance 2 and aresistance 3, the thermistor 1, reactance 2 and resistance 3 being inseries relation to each other. The controlled temperature and its rangeare determined by the voltage-current characteristics of the thermistorand the load line of the external impedance comprised of the reactanceand the resistance.

Referring to FIG. 2, reference characters 4, 5, 6, and 7 designate thecharacteristic curves of said thermistor with regard to various ambienttemperatures there-of, respectively, and 8 represents the load line ofsaid external impedance. A current flowing through the thermistor at acertain temperature increases with an increase in the applied voltageand at the same time the temperature of the thermistor itself goes up.Since the thermistor has a positive temperature coefficient ofelectrical resistance, the current flowing through the thermistordecreases with increasing applied voltage where the temperature ofthermistor exceeds a specified temperature which depends upon theambient temperature and the temperature-resistance characteristics ofthe thermistor. The characteristic voltage-current curves for thetemperatures of the thermistor are curved upwardly and then downwardlyas shown in FIG. 2. When the ambient temperature is higher, thespecified temperature is achieved by a lower current flowing through thethermistor. Therefore, the characteristic curves for the temperature ofthermistor shift down with an increase in the ambient temperature asshown in FIG. 2 where the curve 4 is for a lower limit of thetemperature and the curve 7 is for an upper limit of the temperature. Anupper limit of a controllable temperature range can be a temperature atwhich the characteristic curve of the thermistor is tangent to the loadline of the external im pedance at the peak of the characteristic curveas shown in FIG. 2. A lower limit of a controllable temperature rangecan be a temperature at which the characteristic curve is tangent to theload line at a downward curvature of the characteristic curve. The loadline intersects the characteristic curve for the temperature between theupper limit and lower limit temperature at three points. Of course, theexternal impedance has little temperature sensitivity and the load linethereof is nearly independent of the temperature. Operating pointsrepresented by intersecting points of the characteristic curves of saidthermistor and the load line of the external impedance vary successivelyin the order 9, 10, 11, 12 with increasing temperature, and at the sametime the current flow decreases as shown in the FIG. 2. Since thecharacteristic curve 7 for the upper limit temperature is tangent to theload line at the peak 12, the operating point immediately moves to apoint 13 from a point 12 as soon as the temperature exceeds the upperlimit temperature. Consequently, the current flow drops abruptly. On theother hand, as the temperature goes down from the upper limit of thetemperature range, the operating points move successively in the order13, 14, 15, 16 along the load line 8, and at the same time the currentflow increases. Since the characteristic curve 4 is tangent to the loadline at the point 16 as shown in FIG. 2, the operating point immediatelymoves from a point 16 to another operating point 9 with a furtherdecrease in the temperature. Thus, the current flow increases abruptly.

This combined thermistor and impedance can be used to make a novel selfcontrollable heating device wherein both the thermistor and externalimpedance are used as heating elements.

FIG. 3 shows the temperature-current characteristics of a combinedthermistor and external impedance. In order to clarify the operation,the operating points on the curve in FIG. 3 correspond to the operatingpoints in FIG. 2 and they are designated by the same numbers. As thetemperature rises from the lower limit 19 of the temperature range, theflowing current decreases gradually in the order 9, 10, 11 and 12. Whenthe temperature of the thermistor rises above the upper limit 18 of thetemperature range, the current flow drops abruptly from point 12 topoint 13. This abrupt decrease in the current flow lowers thetemperature. As the temperature goes down the current flow graduallyincreases in the order 13, 14, 15 and 16 with the up-shift ofcharacteristic voltage-current curves for the temperature of thethermistor. When the temperature of the thermistor drops below the lowerlimit 19 of the temperature range, the current flow increases abruptly.Thus, the heating device according to the present invention can controlan ambient temperature.

Referring to FIG. 3, the temperature range 17 between the upper limit 18and the lower limit 19 can be predetermined by a selection ofcharatceristics of said thermistor and the load line of the externalimpedance, such as a reactance and a resistance, because, the upperlimit and the lower limit of the temperature range can be determined bya combination of the charatceristic voltagecurrent curve for thetemperature of the thermistor and the load line of the externalimpedance as explained in the preceding description.

FIG. 4 shows a circuit diagram of a temperature responsive controlsystem wherein reference characters 22, 23 and 24 represent a thermistorhaving a positive temperature coeflicient of electrical resistance andconsisting essentially of barium titanate, an external impedance and acurrent actuated means, such as a solenoid coil and a solenoid switch,respectively. The control action of the control system is brought aboutby an abrupt change in current flow responsive to change of temperaturesto temperatures outside a range of temperatures. The said range oftemperature can be determined by a combination of the characteristicvoltage-current curves for the temperature of the thermistor 22 and theload line of both the external impedance 23 and the current actuatedmeans 24 in a similar way to that explained above in connection withFIGS. 2 and 3.

Furthermore, a temperature sensitive electric switch can be easilyprepared by employing the aforesaid temperature responsive controlsystem.

The temperature sensitive electric switch comprises a thermistor havinga positive temperature coefficient of electrical resistance andconsisting essentially of barium titanate and an external impedanceconnected in series with said thermistor. The said series connectedthermistor and impedance are adapted to be coupled to a source ofcurrent. Referring to FIG. 3, the current flow indicated by point 12 isa critical point at the upper limit 18 of the temperature range and thecurrent flow indicated by point 16 is another critical point accordingto the lower limit 19 of the temperature range. When the temperatureslight- 1y exceeds the upper limit 18 of the temperature range, thecurrent flow decreases immediately to a point indicated by 20 inaccordance with the novel control action of the present invention, andthen the temperature decreases to the lower limit 19 of the temperaturerange. When the temperature drops slightly down below the lower limit 19of the temperature range, the current flow suddenly increases up to apoint 21 and then the temperature increases. Repeated cycles of thistype actuate the current actuated means which can in turn be used tocontrol current flow to and from an apparatus. The control system thusacts as a stable and sensitive thermal switch. This thermal switch isoperable even when there exists a great variation in the appliedvoltage, because a variation in the applied voltage results in avariation in the temperature of thermistor.

The following examples of specific new devices are given by way ofillustration and should not be construed as limitative.

FIG. 5 is a cross sectional view of a milk warming device whichmaintains a substantially constant temperature by using the temperaturesensitive control system of the present invention. It is preferred tohave the temperature of the milk about 50 C. for a baby. In FIG. 5,reference character 25 is a nursing bottle, water 26 is used as aheating medium in container 31 having cover 32, and a coating 27 ofelectrically insulating material such as polytetrafluoroethylene ispositioned over the heating ele ment 28, which element is mounted in thebottom of the container 31 and is prepared by the following procedure.

An equimolar mixture of barium carbonate and titanium oxide isWet-milled with the addition of 2 percent by weight of silver oxide and3 percent by weight of an excess of titanium oxide. The resultantmixture is pressed into a disk 15 cm. in diameter and 0.5 cm. inthickness and fired in flowing nitrogen gas at 1350 C. for 2 hours. Asemiconductive sintered body black in color results, and is heated inair at 1100 C. for 1 hour to increase the positive temperaturecoefficient of electrical resistance. The sin- J tered ceramic disk 8cm. in diameter and 0.3 cm. in thick- 0 ance 30 is connected in seriesto the termistor 28 by an electric lead. The external impedancecomprises a coil 30 having a reactance of IS/Lh. and a resistor having aresistance of 35 ohm. FIG. 6 shows the temperature-current curve for thewarming device illustrated in FIG. 5, and indicates that the centertemperature and range of controlled temperature are 50 C. and :5 C.,respectively in accordance with the present invention. Thesecharacteristics are, of course, independent of the amount of heatedmilk.

It will be understood by those skilled in the art that systems accordingto the invention can be modified in various respects without departingfrom the essence of the invention and within the essential features ofthe invention as set forth in the claim annexed hereto.

What is claimed is:

1. A temperature responsive resistance device responsive to change flowof current therethrough at the ends of a range of temperatures, saiddevice comprising a thermistor having a positive temperature coefficientof electrical resistance and consisting essentially of barium titanate,the characteristic voltage-current curves for the temperatures of thethermistor at the ends of the range of temperatures being curvedupwardly and then downwardly, and an external impedance connected inseries with said thermistor, the voltage-current load line of saidimpedance intersecting the characteristic curve for the lowertemperature of the range adjacent the origin end of the curve and beingtangent to the other end of said last-mentioned characteristic curve,and said load line being tangent to the peak of the characteristic curvefor the higher temperature of the range adjacent the origin end of thecurve and intersecting the other end of said last-mentioned curve, saidseries connected thermistor and impedance being adapted to be coupled toa source of current, whereby the current fiow drops abruptly when thetemperature of the thermistor rises above the upper limit of thetemperature range and the current flow to increase abruptly when the9/1962 Swanson 2l9505 6/1965 Andrich 219--504 RICHARD M. WOOD, PrimaryExaminer.

L. H. BENDER, Assistant Examiner.

